Photochromism is defined as “a reversible transformation of a single chemical species being induced in one or both directions by absorption of electromagnetic radiation, with two states having different distinguishable absorption spectra”. Photochromic compounds are compounds that possess at least two isomeric forms, which have different physical properties, such as absorption and emission properties, refractivity, and the like, and can be transformed from one form to another by photo-excitations at prescribed wavelengths.
Photochromism has been extensively studied due to its potential use for optical recording and other optical functioning devices. To be practically used as optical recording materials, both isomeric forms must be thermally stable and possess excellent durability for reversible photochromic reactivity. Diarylethene is one class of photochromic compounds, which possesses all these necessary properties, and therefore is a suitable class of compounds for the construction of optical functioning devices. The cis-configuration of both aryl groups in the diarylethenes studied is generally fixed by an upper cycloalkane structure, such as fluorinated alicyclic group, aromatic group, anhydride and maleimide group. Apart from the difference in absorption characteristics and the like between the two forms and their thermal stabilities, the availability of desirable excitation wavelengths that can be tuned and selected for the photochromic reactions also represents an important aspect in the design of materials for optical functioning devices.
Even though there has been increasing interest in diarylethene-containing photochromic materials, most efforts have been focused on the derivatization of the diarylperfluorocyclopentenes to tune the photophysical and photochromic behaviors while less efforts have been made in the design and synthesis of different types of diarylethenes with excellent photochromic properties. However, the derivatization of diarylperfluorocyclopentenes has been rather limited with most of the works mainly focused on modifications at the substituted aryl groups only.
The most commonly studied heterocycles include pyrroles, thiophenes, indoles, thiazoles, imidazoles and others. Amongst the many heterocycles, phospholes and siloles have been less extensively studied, but have recently attracted increasing interests due to their unusual electronic and optical properties and possible application as organic light-emitting devices (OLEDs). Recently, Yam and co-workers [Yam, V. W.-W.; Ko, C.-C.; Zhu, N. J. Am. Chem. Soc. 126, 12734 (2004); Yam, V. W.-W.; Lee, J. K.-W.; Ko, C.-C. Zhu, N. J. Am. Chem. Soc. 131, 912 (2009); Wong, H.-L.; Ko, C.-C.; Lam, W. H.; Zhu, N.; Yam, V. W.-W. Chem. Eur. J. 15, 10005 (2009); Poon, C.-T.; Lam, W. H.; Yarn, V. W.-W. J. Am. Chem. Soc. 133, 19622 (2011)] and other research groups [Nakashima, T.; Fujii, R.; Kawai, T. Chem. Eur. J. 17, 10951 (2011); Kühni, J.; Belser, P. Org. Lett. 9, 1915 (2007)] have shown that the incorporation of heterocycles into the “ethene” part of the diarylethene backbone, instead of derivatizing the pendants of the bis(thienyl)perfluorocyclopentene core, can enrich the photochromic and photophysical behaviors. In spite of the increasing interest in the use of phospholes and siloles and their derivatives for the fabrication of OLEDs, there are no examples on the use of functionalized phosphole and silole as the “ethene” part of the photochromic diarylethene backbone.
Further information can be found in U.S. Pat. Nos. 5,175,079, 5,183,726, 5,443,940, 5,622,812, and 6,359,150; Japanese patents JP 2-250877, JP 3-014538, JP 3-261762, JP 3-261781, JP 3-271286, JP 4-282378, JP 5-059025, JP 5-222035, JP 5-222036, JP 5-222037, JP6-199846, JP 10-045732, JP 2000-072768, JP 2000-344693, JP 2001-048875, JP 2002-226477, JP 2002-265468 and JP 2002-293784; and in Irie, M.; Mohri, M., J. Org. Chem. 53. 803 (1988); Nakamura, S.; Irie, M. J. Org. Chem. 53. 6136 (1988); and Irie, M. Chem. Rev. 100. 1685 (2000).